In the image forming apparatus using the electrographic method, a toner image is formed on an image carrier of a rotating photoreceptor drum or a photoreceptor belt, the formed toner image is transferred onto a recording sheet directly or indirectly, and the toner image is fixed to form an image.
In the above-mentioned image forming method, the toner image electrostatically transferred onto the recording sheet is stably fixed on the recording sheet by heat and pressure of a fixing roller. The fixing roller surface may be scratched or grooved by the edge of a recording sheet often touching the same part of the fixing roller surface. This situation will cause the problem that the service life of the fixing roller becomes short.
In order to prevent such scratching and grooving, there are measures of shifting the fixing roller in an axial direction. By this shift of the fixing roller, the position at which the edge of a recording sheet touches the fixing roller is varied, and scratches and grooves will be reduced. Whereby, the problem that the service life of the fixing roller is shortened will be resolved.
When the fixing roller is shifted in such a way, the temperature gradient gets large and the temperature distribution on the fixing roller gets uneven due to individual variations of parts and depending on the accuracy variation of assembly, thereby causing insufficient fixing and insufficient gloss of image, resulting in insufficient image quality.
An image forming apparatus employing such a reciprocal shift is proposed following Japanese Laid-Open Patent Application Publication No. 2006-133581 and the like.
In Japanese Laid-Open Patent Application Publication No. 2006-133581, it is determined based on information of temperature whether the location of the recording sheet is appropriate. Thus, the temperature is not actively controlled for the fixing device. Therefore, when the aforementioned situation such as insufficient fixing, insufficient gloss of image, and uneven gloss has occurred, it cannot be surely prevented.
In recent years, there is an image forming apparatus using two fixing devices, a first fixing device and a second fixing device, in order to realize sufficient fixing. In such an image forming apparatus as well, the reduction of the service life due to scratching and grooving on the fixing roller surface is a problem. However, even if the two fixing devices are simply shifted, the imaging quality may be reduced due to a temperature gradient.